In conventional scanning electron microscopes (SEM), the specimen chamber is typically maintained at a vacuum pressure of 0.01 Pa or lower. This allows maintaining a sufficiently low pressure level in the electron gun and also to use so-called “in-lens” or “through-the-lens” detector systems, which are typically disposed inside the particle optical column. These low pressure levels also prevent degradation of the image quality, which may occur due to collisions of primary beam electrons with residue gas particles.
However, the requirement of maintaining the specimen chamber at a high vacuum makes it difficult to inspect wet or non-conductive specimens, such as biological materials, plastics, ceramics, minerals and fibers. Wet specimen deteriorate the vacuum pressure level by outgassing. For non-conductive specimen, the low vacuum pressure level prevents dissipation of surface charges that accumulate on the surface.
To enable inspection of wet or non-conductive specimens, preparation techniques, such as drying, freezing or vacuum coating have been developed. These techniques, however, are often not desirable, since they tend to alter or mask the sample surface.
Attempts to overcome these constraints have led to the development of special kinds of scanning electron microscopes, such as variable-pressure scanning electron microscopes (VPSEMs) and environmental scanning electron microscopes (ESEMs). These types of scanning electron microscopes can be operated at elevated gas or vapor pressure levels in the specimen chamber, which can be up to 2500 Pa in the case of ESEMs. Operation at these elevated pressure levels is made possible by one or more differential pressure apertures, which are provided to limit the amount of gas in the electron optical column.
However, it has been shown that these technologies are not fully compatible with through-the-lens detector systems, since the additional differential pressure apertures often reduce the amount of backscattered electrons and secondary electrons, which pass through the objective lens, and hence, the detected electron intensity. Further, the additional differential pressure apertures typically limit the attainable field of view.
Accordingly, there is a need to provide a particle optical system, which allows efficient inspection of a wide range of objects.